The Solubility and Sorption of Lead-210 and Carbon-14 in a Near-Field Environment

Abstract

This paper reports the results of some recent experimental studies of the solubility and sorption behaviour of lead-210 and carbon-14 under cementitious near-field conditions.

These studies have shown that under these conditions carbon-14 will have a maximum solubility limit of 10⁻⁴ M and that the distribution ratio, R_D, will increase with increasing carbon-14 concentrations from 10⁻⁹ to 10⁻⁷ M. Not all of the carbon in the cement is available for exchange with carbon in the pore water. Differences in values of R_D are observed between the two cement grout types studied, SRPC and OPC/BFS. Lead has been shown to have a maximum solubility limit of about 10⁻³ M at high pH. Good agreement is obtained between these measurements and thermodynamic modelling using the PHREEQE code. No differences were observed between lead solubilities under reducing or oxidising conditions at high pH values using the same phase separation techniques. Lead is particularly sensitive to the phase separation techniques employed. A factor of up to 250 difference is observed between 25000 and 30000 molecular weight cut-off filters. The values of R_D for lead increase with decreasing lead concentrations and the values of R_D for 10⁻³ M solutions are observed to be 500 mlg⁻¹ for SRPC and 1300 mlg⁻¹ for OPC/BFS.